We'll want to pass in arguments to render (i.e., width),
so using __str__ is not going to be appropriate.

a) don't use format, use __str__, since nothing calls
   format directly
b) don't declare interest in keys; just don't compute
   rendered value in update method, only when actually
   rendering (in __str__)
c) don't pass in the available width to update, since
   that is only relevant during rendering (this will
   need to be fixed later, currently we don't get width
   at all)

Add a format() method that actually do the heavy duty lifting,
such as string formatting, which is only needed during actual
output.